Digital images are now widely available for various types of imager devices that may use CCD, CMOS or other types of pixel arrays and associated readout circuits. Most digital images are constructed from the analog signals output from the pixel array, typically, though not exclusively, output as Bayer pattern analog signals. The analog pixel signals, each of which represents one color, e.g., red, green or blue, are converted to digital signals that are processed to produce demosaiced pixel signals forming a digital image, which may be stored, transmitted, and/or further processed.
Noise is an image distorting feature that may be present in a stored, transmitted or processed digital image and may come from different sources. For example, there may be light noise produced by the pixel array and associated readout circuitry, such noise is intensified by continued decreases in pixel size, and under low light conditions. In addition, quantization errors may occur in the analog-to-digital converter that digitizes the signals. Noise may also be introduced by a recording medium that stores the digital image or by a transmission medium.
As a consequence, digital image denoising has become an important part of digital image processing. In many instances, digital images are processed for noise content in the later stages of digital image processing after a demosaicing operation on the original pixel array signals. In such cases, noise present in the initial digitized image may be further intensified by early stage digital processing and as a result may become more difficult to remove, requiring a higher degree of noise processing, which may excessively distort or blur the digital image. In addition, performing a denoising operation after demosaicing requires a large line buffer memory, which increases the size of a chip containing an image processor.
What is desired, then, is a method and apparatus for denoising prior to or during demosaicing.